It has long been known that combustion efficiency and exhaust emissions can be improved at some engine operating conditions by gradually increasing the mass flow rate of fuel injected into the combustion chamber during an injection event. Two of these injection profiles are oftentimes referred to in the art as a ramp or boot shaped injection. In the field of hydraulically actuated fuel injectors, this type of rate shaping can be accomplished in a number of ways. One method is by controlling the initial velocity profile of a plunger that pressurizes the fuel during each injection cycle. The movement of the plunger in a hydraulically actuated fuel injector can in turn be controlled by controlling the movement rate of the piston, which supplies the downward force to the plunger.
One known method for rate shaping involves creating an initial hesitation in the piston. This can be accomplished by designing geometrical relationships between the piston and the piston bore that prevent the high pressure hydraulic fluid from acting over the complete surface of the piston when the piston begins its downward movement. One example of this method is described in U.S. Pat. No. 5,522,545 to Camplin et. al. In Camplin et al, the geometrical relationship between the cavity and the piston prevents the full pressure of the actuation fluid from acting over the whole area of the piston when the injection event begins, causing the piston to hesitate in its initial downward movement.
In another example, U.S. Pat. No. 3,921,604 to Links describes a fuel injector having an intensifier piston with a conical protuberance on its top side that projects into the high pressure hydraulic fluid supply bore. Links describes this geometry as giving the injector the ability to control the stroke speed of the piston, presumably because the conical portion prevents the high pressure fluid from flowing quickly to act on the remaining surface area of the piston. While Links does recognize that some injection rate shaping capability can be accomplished by the geometrical interrelationship between the piston and the high pressure hydraulic fluid supply bore, the Links geometry suffers from a number of disadvantages which render it difficult to reliably predict performance due to extreme sensitivity to machining tolerances.
The present invention is directed to overcoming one or more of the problems as set forth above.